A highly efficient procedure for transferring functional genes into mammalian cells has been developed using retroviral vectors as a delivery system. Retroviral vectors have been built which contain one of a number of different human (or other) genes. Clinical protocols have been prepared using retroviral-mediated gene transfer as a means of studying or treating human diseases. A human gene transfer protocol using NeoR-gene marked TIL to study adoptive immunotherapy for malignant melanoma is being successfully carried out. Two gene therapy protocols have been approved: one for the treatment of patients suffering from ADA Deficiency by inserting the human ADA gene into the patient's own T lymphocytes and the other for inserting a TNF gene into TIL cells as an addition to TIL adoptive immunotherapy for the treatment of advanced cancer.